A production switcher is used in a television studio to switch selectively between different television signal sources in the production of a final program video signal. Generally, the audio portion of the television signal is separated from the video portion upstream of the production switcher and is recombined with the video portion downstream of the production switcher. The audio portions of the various television signals are combined or selected using a separate audio mixer, because it is not desirable that the audio signal be produced in the same fashion as the video signal. For example, it is quite acceptable to have a rapid switch between two video signals, in an interval on the order of tens of nanoseconds, but if the source of an output audio signal is changed in such a short time, switching transients in the output audio signal will manifest themselves as audible clicks when the audio signal is used to drive a loudspeaker. It is therefore desirable that the switch between a first audio input signal and a second audio input signal should take place over a longer time, for example on the order of a few milliseconds.
In an audio mixer, it is conventional for the audio input signals to be applied to respective voltage controlled amplifiers and for the output terminals of the VCAs to be connected to the output terminal of the mixer. The gain of each VCA is controlled by applying a control signal to its control input terminal. In order to switch between a first audio input signal and a second audio input signal, the control voltage applied to the VCA in the first signal channel is decreased while the control voltage applied to the VCA in the second signal channel is increased. However, if a linear ramp is used as the control voltage, the discontinuity at the start and end of each ramp generates a switching transient on the audio output signal that may be observed as a click. In order to eliminate these clicks, it is necessary to round both ends of the ramp so as to eliminate substantial discontinuities.
It is known to use passive filters constructed of reactive circuit elements to shape the waveforms of control signals that are used in a video production switcher. However, such passive filter devices are not suitable for use in an audio mixer because of the much longer time constants involved.